The present invention relates to an antenna having a flat radiowave radiating surface and, more particularly, to a planar antenna suitable for use in combination particularly with a terminal apparatus included in a radio communication system.
Antennas for radio apparatuses using frequencies in a microwave frequency band or a millimeter wave frequency band are array antennas having an array construction which enhances the gain to secure a satisfactory quality of communication using radio waves of short wavelengths. The array antennas include microstrip array antennas, such as disclosed in, for example, Japanese Patent Laid-open (Kokai) Nos. Hei 1-269302 and Hei 1-292903, having a feeder line and radiation elements arranged in a plane,a triplet micro-strip antenna, such as disclosed in, for example, Japanese Patent Laid-open (Kokai) No. Hei 4-82405, having a feeder line formed in an inner layer to feed power to radiation elements formed in an outer layer, and waveguide planar antenna, such as disclosed in, for example, Japanese Patent Laid-open (Kokai) No. Hei 1-314405, having an array of a plurality of rectangular waveguides having upper walls provided with slots which serve as radiation elements to reduce loss by a feeder line, and receiving power.
If the number of the radiation elements of the prior art micro-strip array antenna is increased to enhance the antenna gain, the number of branches of the feeder line for feeding high frequency power to radiation elements increases and feeder loss attributable to multiple reflection by the branches increases. Therefore, it is difficult to enhance the antenna gain by increasing the number of the radiation elements. The prior art waveguide planar antenna needs much time and labor for fabrication, because walls of a length equal to several times the wavelength must be formed perpendicular to the surface of the planar antenna to realize an electromagnetic mode for the waveguides. Therefore, it is difficult to mass-produce the waveguide planar antenna and the waveguide planar antenna is inevitably costly despite of various proposals.